Many instruments and systems are limited in sensitivity or bandwidth by noise in the electromagnetic field measured by such instruments or utilized in such systems. For example, coherent communications systems typically distinguish signals which are 180.degree. apart in phase, and instruments such as interferometers and ring gyroscopes rely on the comparison of small phase differences between electromagnetic signals which correspond to the physical quantities they measure. Uncertainties in phase due to noise in such devices limit their accuracy and utility. Although many sources of noise may be eliminated by proper design, a fundamental limit is imposed by the requirements of quantum mechanics, in that zero point fluctuations of the electromagnetic field (present even at zero temperature) cannot be eliminated. Thus, the electric field of a single electromagnetic mode of angular frequency .omega. can be written as EQU E=E.sub.o (X.sub.1 cos .omega.t+X.sub.2 sin .omega.t),
where E.sub.o is a constant containing the amplitude of the field, and X.sub.1 and X.sub.2 are real-valued quantities known as field quadrature operators. The Heisenberg uncertainty principle requires that the variances, .DELTA.X.sub.1 and .DELTA.X.sub.2, of such operators are related by the uncertainty relation EQU .DELTA.X.sub.1 .DELTA.X.sub.2 .gtoreq.1/4.
Although .DELTA.X.sub.1 and .DELTA.X.sub.2 may be unequal in principle, for devices known to the present art (such as single mode lasers), these variances are typically equal and hence take the minimum values .DELTA.X.sub.1 =1/2, .DELTA.X.sub.2 =1/2. This minimum uncertainty in the quadrature operators is in turn reflected in minimum uncertainties in measurements of the electric field, E. For example, if E were to be measured at periodic intervals corresponding to .omega.t=N.pi., where N is an integer, only the first quadrature term would contribute, and such measurements would therefore be subject to the variance of X.sub.1. Conversely, if measurements were made at times such that .omega.t=(N+1/2).pi., where N is an integer, only the second term would contribute and such measurements would be subject to the variance of X.sub.2. So long as the variances are equal, no advantage in the accuracy of such measurements is possible by selecting either X.sub.1 or X.sub.2.
In principle, however, it is possible to construct fields E such that the variances of X.sub.1 and X.sub.2 are unequal, e.g., that .DELTA.X.sub.1 &lt;1/2 and .DELTA.X.sub.2 &gt;1/2 while preserving the product rule .DELTA.X.sub.1 .DELTA.X.sub.2 .gtoreq.1/4. It has been recognized for some time that such states of the electromagnetic field, called "squeezed states", are permitted by theory. See, for example, the review by D. F. Walls in Nature, Vol. 306, pp. 141-146 (1983). Providing that such squeezed states can be realized in a working device, the possibility of improving accuracy in devices such as interferometers has been noted by a number of authors; e.g., C. M. Caves, in Physical Review, Vol. D23, pp. 1693-1708 (1981), and R. S. Bondurant and J. H. Shapiro, in Physical Review, Vol. D30, pp. 2548-2556 (1984).
Many phase-dependent nonlinear phenomena have been suggested for squeezed state generation, as described by Walls in the above-cited review article. In particular, H. P. Yuen and J. H. Shapiro, in Optics Letters, Vol. 4, pp. 334-336 (1979), suggested the interaction of four electromagnetic waves ("four-wave mixing") as a generation mechanism. Four-wave mixing has typically been described in connection with so-called "degenerate" mixing, in which all four waves have a common (angular) frequency .omega..sub.p, the frequency of the "pump" used to provide the coherent input waves (typically a laser source). The use of an optical cavity to enhance the generation process has been suggested by B. Yurke, in Physical Review, Vol. A29, pp. 408-410 (1984). However, it has proven difficult in practice to achieve the conditions necessary to produce and measure squeezed states. In particular, a problem with the approach based on degenerate four-wave mixing has been extraneous noise generated, for example, by spontaneous emission or scattering from the (pumped) mixing medium at frequencies equal to or nearly equal to the pump frequency. Such extraneous noise adds to the quantum noise which must be squeezed to reach the quantum limit.